The reaction between olefins,N-chloroacetamide or 1,3-dibromo-5,5-dimethylhydantoin, and hydrogen peroxide affords β-halohydroperoxides in good yields. These compounds react extremely rapidly in basic solution to give products which depend upon their structure. The 3-halo-2,3-dimethyl-2-butyl hydroperoxides form 2,3-dimethyl-3-hydroperoxy-1-butene and from the 2-halo-1,2-dimethylcyclohexyl hydroperoxides 1-methyl-2-methylene-cyclohexyl hydroperoxide is obtained. No allylic hydroperoxide can be detected from the reaction between base and 3-bromo-2-methyl-2-butyl hydroperoxide. Mainly cleavage products, acetone and acetaldehyde, are formed together with some 2,3-epoxy-2-methylbutane. The reaction between base and 2-bromo-1-phenylethyl hydroperoxide gives styrene oxide, benzoic acid, and some benzaldehyde. Fromtrans-2-bromocyclohexyl hydroperoxide,trans-2-bromocyclohexanol, cyclohexene oxide, and sometrans-1,2-cyclohexanediol are obtained upon reaction with base. No evidence for the formation of the allylic hydroperoxide was obtained.